1. Field of the Invention
The invention relates to a system for the monitoring and wireless signaling of a pressure or a change of pressure in pneumatic tires on vehicles.
2. Description of Related Art
Prior art systems for monitoring pressure in pneumatic tires comprises a device with an electronic circuit, hereinafter referred to as wheel electronics, disposed inside the pneumatic tire and exposed to the pressure prevailing in that tire. The wheel electronics comprise a battery as a power source, a pressure sensor for measuring the pressure prevailing in the pneumatic tire, a transmitter for transmitting information relating to the pressure or the change of pressure, and a receiver for receiving wireless interrogation and/or control signals.
Given the fact that battery changes are either impossible with such wheel electronics or possible only at significant cost, it is desirable to operate the wheel electronics in the best energy-saving fashion possible. Especially, it is desired to have a battery whose service life is as far as possible identical to the service life of the vehicle. It is one object of the present invention to provide wheel electronics with a long life. If the wheel electronics are caused to emit pressure or pressure change signals by wireless interrogation and/or control signals emitted by an interrogation transmitter, it can be ensured that the especially energy-intensive transmission of information on the prevailing pressure or change of pressure is performed only at times when such information is really needed. It is then possible to provide, for example, that no pressure data are transmitted when the vehicle is parked, and that the transmission rate is adapted to the vehicle speed. By controlling the wheel electronics via interrogation and/or control signals it is possible in this way to achieve especially energy-saving operation of the wheel electronics. The use of a lithium battery, having the dimensions of a button cell, then permits service lives of seven years or over to be achieved.
Apart from the desire to achieve the best possible energy-saving operation, the clear and—when a tire has been changed—rapid association of the wheel electronics to the position of their associated wheel on the vehicle presents a permanent problem. DE 197 35 686 A1 proposes for this purpose to have the wheel electronics simultaneously detect and evaluate the path acceleration and the Korreolis acceleration of the respective wheel. That publication explains in detail that it is possible in this way to differentiate between steered front wheels and unsteered rear wheels, as well as between left wheels and right wheels. Together with measuring signals relating to the pressure or a pressure change, the path acceleration and the centrifugal acceleration, each wheel electronics transmit in this case also an identifying signal characteristic for it. Using the respective identifying signals, an evaluation circuit arranged in or on the vehicle can then automatically associate the different wheel electronics to the respective wheel positions by statistical evaluation of the centrifugal acceleration and the path acceleration. When a wheel has been mounted on the vehicle for the first time, or else after a change of a wheel, the evaluation unit therefore automatically learns which wheel electronics belong to which wheel position.
However, it is a disadvantage of the method disclosed by DE 197 35 686 A1 that following a change of a tire, for example, several minutes of driving are required until it can be derived from the centrifugal and path acceleration values measured which identifying signal relates to a given wheel position.